6-DoF tracking, also known as six degrees of freedom tracking, is a method by which a device (e.g. mixed-reality head-mounted device (HMD), robot, smartphone, etc.) uses sensors (e.g. cameras, inertial measurement units, etc.) to determine its position relative to its surrounding physical environment. For example, a mixed-reality HMD or smartphone can use this positional understanding to place holograms or digital content so as to appear to be world-locked to a position in the physical world, and a robot can use this positional understanding to navigate itself relative to its surroundings. Recently, scenarios have arisen in which it is useful to have two or more such devices operating with a common understanding of their positions relative to a physical environment, and thus relative to each other. As discussed in detail below, there are several general approaches to developing this common understanding of positions between such devices, each with significant challenges recognized by the inventors.